1. Field of the Invention
The present invention relates to remote instrument monitoring systems. In particular, the present invention is an improved transponder and interrogate/receiver for use in a remote RF instrument monitoring system.
2. Description of the Prior Art
Commodities such as gas, water, and electricity have been tradionally monitored by meters physically located at the consumer's facility or residence. The sight of meter reading personnel walking from door to door and recording by hand the accumulated meter reading is a common one with which nearly everyone is familiar. Although this meter reading technique is traditional, it is inefficient, susceptible to error, requires many employees, and is very expensive.
Apparatus and methods for automatically communicating data from a plurality of remotely located parameter sensing instruments, such as commodity meters, to a central data acquisition system have, in fact, been developed. One such system is disclosed in a patent application entitled AUTOMATIC/REMOTE RF INSTRUMENT READING METHOD AND APPARATUS (hereinafter referred to as Instrument Reading Apparatus) which is identified in the United States Patent and Trademark Office by Ser. No. 06/703,621 issued on Sept. 30, 1986 as the Brunius et al. U.S. Pat. No. 4,614,945, and assigned to the same assignee as the present invention. The Instrument Reading Apparatus disclosed therein includes a plurality of transponders. or Encoder/Receiver/Transmitters (ERTs), one of which is associated with each remotely located meter or instrument. Also included is an interrogate/receiver, which can be included within a mobile data acquisition system. The interrogate/receiver transmits a "wake-up" or activation signal. All transponders then within range of the interrogate/receiver wake up and initiate transmission of an RF transponder signal which includes account data representative of the parameter sensed by a particular meter with which it is associated. The interrogate/receiver simultaneously receives the transponder signals from all activated transponders, and stores the account data contained therein. Account data is later removed and used for utility billing purposes.
In the Instrument Reading Apparatus, the transponder signals are comprised of a series of spaced transmission bursts, each of which includes the account data. In order to reduce the probability of transmission collisions, i.e., the simultaneous transmission of a transponder signal from two or more transponders at the same time and/or at the same frequency, the transponder signal is characterized by active time and/or frequency parameters. Each transponder causes the frequency at which the transmission bursts of a transponder signal are transmitted to very so as to occur at different frequencies within a predetermined bandwidth. In addition, the spacing in time between transmission bursts of different transponders vary, although the spacing in time between transmission bursts of any given transponder is constant.
Although the active time and/or frequency parameters utilized by the Instrument Reading Apparatus significantly reduce transmission collisions between simultaneously activated transponders, they do not do so to the extent required of a commercially viable product. Transmission collisons still occur with enough regularity to prevent reliable data communication with the interrogate/ receiver at economically feasible rates.
Another problem with the Instrument Reading Apparatus described above concerns the accuracy of data communications between the transponders and the MDAS. All data communication systems, especially digital RF systems such as that described above, can be characterized by a statistical probability of error. Despite this fact, error detection techniques implemented by the Instrument Reading Apparatus are quite limited. They include determining whether the preamble received has the proper sequence of digital values, and whether the correct number of bits have been received. Even if these techniques indicate receipt of a "valid" transmission, there is apparently no way to determine if the encoded data representing the meter reading was valid, i.e., received as transmitted.
Yet another very important feature of a commercially viable instrument monitoring system is the length of time that it can operate without requiring a new supply of power such as that provided by batteries. The instrument monitoring system described above activates the transponders by an activation signal in the form of an RF carrier of predetermined frequency. Various communication services operating within the same frequency range as the carrier cause a certain amount of falsing, accidentally walking up the transponders. Accidental wake-ups initiate the transmission of the transponder signal, and thereby waste battery life.
It is evident that there is a continuing need for improved automatic/remote RF instrument monitoring systems. To be commercially viable, the system transponders must meet several requirements. First, the transponder must be capable of producing collision resistant transmissions. Active time and/or frequency parameters which result in transponder signals with collision resistant characteristics superior to those of known techniques must be developed. A transmission protocol capable of accurate transmission is also required. The protocol must provide the capability for detecting errors in the transmitted data representative of the sensed parameter. The transponders should also be resistant to false wake-ups. These and other characteristics must be achieved with a relatively inexpensive transponder which is highly reliable.